Signal-processing method and active sonar implementing same

ABSTRACT

The invention relates to the field of underwater acoustics and more particularly to the field of signal processing in a low frequency (LF) active sonar system. The present invention makes it possible to decrease the false alarm rate while retaining the classification of the objects. The subject of the invention is a method of processing signals received corresponding to a signal emitted comprising by recurrence two pulses, a first Doppler tolerant broadband pulse of HFM type in particular and a second Doppler intolerant broadband pulse of BPSK type in particular, comprising: a step of detecting objects performed on the part of the signal received corresponding to the first pulses and providing an alarm for each object detected, and a step of classifying the objects detected performed on the part of the signal received corresponding to the second pulses for the alarms satisfying at least one predetermined criterion.

FIELD OF THE INVENTION

The invention relates to the field of underwater acoustics and moreparticularly to the field of signal processing in a low frequency (LF)active sonar system.

BACKGROUND OF THE INVENTION

This type of system is generally towed from a surface vessel andcomprises a fish equipped with an LF emitter which tows a linearreceiving antenna furnished with acoustic or hydrophonic sensors. Such afish and such an emitter are for example described respectively inFrench Patents published under numbers 2735645 and 2776161. However, theinvention may be applied to all types of active sonars. It is well knownthat an active sonar emits recurrent acoustic pulses and that the echoesreceived in return are processed to detect and classify any targets.

When an active sonar operates in a zone such that the bottom isinsonified, the reverberation which originates from the bottom inessence greatly limits the operational usefulness of the sonar onaccount of the overly large number of false alarms which appear. This isparticularly true for shallow depths.

To reduce nuisance in a reverberating medium, it is known to useemission codes that harness the wide frequency bands, typically anoctave, of present-day transducers. These codes possess good distanceresolution, hence the large number of alarms that are produced.

It is known to emit at each recurrence, either an HFM (HyperbolicFrequency Modulation) code, or a BPSK (Binary Pulse Shift Keying) codeor an FP (Frequency Pulse) code.

The HFM code is Doppler tolerant: it therefore does not allowmeasurement of the Doppler induced by a target in motion but, on theother hand, the matched filtering on reception requires only a singlecopy.

The BPSK code is Doppler intolerant and is used to measure the Doppler;it allows the same detection performance as the HFM code but the matchedfiltering on reception requires a significant number of copies to carryout the matched filtering on reception, typically a number greater than200, and hence a correspondingly large processing cost.

OBJECT OF THE INVENTION

As for the FP code, it is used to measure the inherent Doppler of theemitter.

SUMMARY OF THE INVENTION

The present invention makes it possible to decrease the false alarm ratewhile retaining the classification of the objects.

The object of the invention is therefore a method of processing signalsreceived corresponding to a signal emitted comprising by recurrence twopulses, a first Doppler tolerant broadband pulse and a second Dopplerintolerant broadband pulse, comprising:

-   -   a step of detecting objects performed on the part of the signal        received corresponding to the first pulses and providing an        alarm for each object detected, and    -   a step of classifying the objects detected performed on the part        of the signal received corresponding to the second pulses for        the alarms satisfying at least one predetermined criterion.

At each recurrence, the two HFM and BPSK codes are emitted. Thedetection of the alarms is done with the HFM code and the estimation ofthe Doppler is done with the BPSK code on the alarms which exceed acertain threshold, so as to eliminate the bottom echoes. Statedotherwise:

-   -   detection with the HFM code    -   Doppler classification with the BPSK code.

Moreover, the bottom echoes being identified, the measurement of theinherent Doppler of the emitter is done by analyzing the bottom echoesproduced by the BPSK code.

DESCRIPTION OF THE FIGURES

The characteristics and advantages of the invention will become moreclearly apparent on reading the description, offered by way of example,and the figures pertaining thereto which represent:

FIG. 1, the successive steps of the method according to the invention,

FIG. 2, the probability distributions of the measured Doppler d_(m) fortwo hypotheses: H₀ for (stationary) bottom echo and H₁ for assumed trueDoppler echo d_(i).

DETAILED DESCRIPTION

FIG. 1 represents the successive steps of the method according to theinvention.

In a known manner, the hydrophone signals undergo upstream processing(demodulation, filtering, amplification, etc.) and are then digitized.In the case of an active sonar, these signals contain the signalsemitted after propagation through the water via the direct path and thereflected paths to which are added the reverberated signals. Inparticular, among the signals reflected, the echoes originating from thesea bottom constitute a significant source of false alarms, inparticular at shallow depths.

According to the invention, at each recurrence are emitted two codedpulses, HFM and BPSK, whose characteristics make it possible to separatethem on reception. They can be emitted at different instants withtotally or partly overlapping frequency bands, or else be emittedsimultaneously in distinct frequency bands, or both at once.

Referring to FIG. 1, the processing of the hydrophone signals consistsfirstly in forming channels S1 in a known manner, this processing beingindependent of the code emitted.

To the signals of HFM channels is applied the matched filteringprocessing S2 consisting in correlating the signal received with a copyof the signal emitted which after rms detection provides signalsrepresentative of the energy as a function of channel (v) and of time(t) i.e. E_(HFM)(v,t).

The next step S3 consists in detecting and in sorting the alarms withregard to an energy criterion. In a conventional manner, the localmaxima are firstly searched for by comparison with a predeterminedthreshold. Thereafter, a normalization is performed by calculating foreach local maximum a value equal to (E_(HFM)−M)/σ where M is the mean ofthe reference noise, taken in the neighborhood of the “channels/time”space (v,t) and σ the corresponding standard deviation. Then, any maximaaround each maximum are eliminated if they have lower normed energy.Finally, the actual detection is obtained by comparing the noneliminatedmaxima with a normed energy threshold.

According to the invention, the matched filtering processing S5 on the“BPSK” channel signals is performed only on the alarms arising from theprocessing of the HFM pulses S4. The matched filtering processingcorresponding to the BPSK code which is Doppler tolerant requires thatthe channel signal be correlated with several Dopplerized copiescovering a range of given target velocities. Thus for an alarm areobtained as many signals as there are copies and form the Dopplerchannels.

The next step S6 consists in estimating the Doppler d and the associatedstandard deviation σ_(d) _(i) of the alarm “i” on the basis of thesignals arising from the Doppler channels. If d_(channel) is the Dopplergiven by the channel in which the alarm is to be found, the Doppler d isobtained by interpolation with the Dopplers of the adjacent channels.

The next step S7 consists in estimating the inherent Doppler d_(p) dueto the velocity of the antennas, emission and reception, with respect tothe bottom. It is estimated at each instant, either on the basis of aDoppler of the echoes originating from the bottom and detected by theBPSK code, or on the basis of the reverberation spectrum obtained by anFP code emitted with the HFM and BPSK codes. The standard deviationσ_(d) is also estimated.

The next step S8 consists in deciding whether this alarm corresponds toa bottom echo or indeed to a true echo at non zero radial velocity. Thevalues of the Doppler di and of the inherent Doppler dp and also thecorresponding rms deviations σ_(d) _(i) and σ_(d) _(p) are available.

Represented in FIG. 2 are the probability distributions of the measuredDoppler d_(m) for two hypotheses: H₀ for (stationary) bottom echo and H₁for assumed true Doppler echo d_(i). H₀ is centered on d_(p) with a rmsdeviation (σ_(d) _(p) ²+σ_(d) _(p) ²)^(1/2) and H₁ is centered on d_(i)with a rms deviation σ_(d) _(i) .

To decide, d_(i)−d_(p) is calculated and a threshold S is chosen: ifd_(i)−d_(p)>S, there is a true echo. The value of S is obtained on thebasis of the values of P_(f) which is the probability of decidingwrongly that a bottom echo is true.

The process of discrimination between true echo with non zero radialvelocity and bottom echo for each alarm detected by HFM is repeated.Next, among the HFM alarms detected and sorted, one undertakes theelimination S9 of the alarms which correspond to the bottom echoes (orto true echoes with zero radial velocity).

In step S10 is obtained an image of the tracks (series of alarms as afunction of time and direction) which is ridded of the false alarms andin particular the bottom echoes, all the better when they are strong andhence a nuisance.

1-13. (canceled)
 14. A method of processing signals receivedcorresponding to a signal emitted comprising by recurrence two pulses, aDoppler tolerant broadband pulse and a Doppler intolerant broadbandpulse, said method comprising: detecting objects performed on the partof the signal received corresponding to the first pulses and providingan alarm for each object detected, and classifying the objects detected;wherein said classification of the objects detected is performed on thepart of the signal received corresponding to the Doppler intolerantpulses for the alarms satisfying at least one predetermined criterion.15. The method of processing signals as claimed in claim 14, wherein thepredetermined criterion applied to the alarms comprises a comparison ofthe alarms with a predetermined threshold.
 16. The method of processingsignals as claimed in claim 15, further comprising a step of firstmatched filtering of the part of the signal received corresponding tothe Doppler tolerant pulses before the detection of objects providing anenergy E_(HFM)(v,t).
 17. The method of processing signals as claimed inclaim 16, wherein the first matched filtering comprises: correlating thepart of the signal received corresponding to the Doppler tolerantpulses, a step of rms detection of the correlated signal providingsignals representing the energy as a function of channel and timeE_(HFM)(v,t).
 18. The method of processing signals as claimed in claim14, wherein said step of detecting objects comprises: searching for thelocal energy maxima E_(HFM)(v,t) by comparison with a predeterminedenergy threshold E_(s), normalizing the maxima obtained by calculationfor each local maxima of the value (E_(HFM)−M)/σ, M being the mean ofthe reference noise and σ the corresponding standard deviation,eliminating the maxima of lower normed energy, selecting the alarmscorresponding to normed maxima not eliminated greater than apredetermined threshold of normed energy E_(SN).
 19. The method ofprocessing signals as claimed in claim 17, wherein said step ofdetecting objects comprises: searching for the local energy maximaE_(HFM)(v,t) by comparison with a predetermined energy threshold E_(s),normalizing the maxima obtained by calculation for each local maxima ofthe value (E_(HFM)−M)/σ, M being the mean of the reference noise and σthe corresponding standard deviation, eliminating the maxima of lowernormed energy, selecting the alarms corresponding to normed maxima noteliminated greater than a predetermined threshold of normed energyE_(SN).
 20. The method of processing signals as claimed in claim 14,further comprising a step of estimating Doppler d_(i) of the alarms icorresponding to the Doppler intolerant pulses for the alarms satisfyingat least one predetermined criterion, and/or the associated standarddeviations σ_(d) _(i) .
 21. The method of processing signals as claimedin claim 20, wherein the inherent Doppler is estimated at each instant:either on the basis of a Doppler of the part of the signal receivedcorresponding to the reverberation of the Doppler intolerant pulses, oron the basis of the spectrum of the reverberation of the part of thesignal received corresponding to the pulses FP when pulses FP have beenemitted.
 22. A method of processing signals received corresponding to asignal emitted comprising by recurrence two pulses, a Doppler tolerantbroadband pulse and a Doppler intolerant broadband pulse, said methodcomprising forming a first channel comprising the part of the signalreceived corresponding to the Doppler tolerant pulses, and a secondchannel comprising part of the signal received corresponding to theDoppler intolerant pulses, first matched filtering of the first channelbefore the detection of objects, detecting the objects providing analarm for each object detected, selecting the alarms satisfying at leastthe predetermined criterion in the second channel, second matchedfiltering of the second channel around the alarms selected, Dopplerestimation of the alarms selected in the second channel, estimatinginherent Doppler, the classifying of the objects by discriminationbetween the bottom echoes and the true echoes on the basis of the valuesof the Doppler of the alarms selected in the second channel and of theinherent Doppler, eliminating on the first channel the alarms detectedcorresponding to bottom echoes.
 23. The method of processing as claimedin claim 22, using jointly signals coming from a pulse of HFM type, andcoming from a pulse of BPSK type, the two pulses being emitted in thesame recurrence.
 24. An active sonar comprising, means of emitting asignal comprising by recurrence two pulses, a Doppler tolerant broadbandpulse and a Doppler intolerant broadband pulse, and means of receivingthe signal emitted implementing the method of processing signals asclaimed in claim
 23. 25. The active sonar as claimed in claim 11,wherein the means of emission emit the two pulses at different instantswith totally or partly overlapping frequency bands.
 26. The active sonaras claimed in claim 11, wherein the means of emission emit the twopulses simultaneously with distinct frequency bands.